That is a great find, but one thing that stands out to me about this sketch is that the main focus is on the head and facial features, with the rest of the sketch just there to frame the important stuff. In consequence, the horn and pouch might just be a generic "horn and pouch" as envisioned by the artist, rather than a faithful if minimalistic attempt to capture Colonel Lewis's actual bag and horn.
I think it is likely to be one of those fingerwoven wool bags, due to the fringe on the bottom. I'd have to look, but I don't think that surviving leather bags from this area and time period typically have fringe on the bottom. The quillwork on the flap looks like a leather bag though, so I wonder if Trumbull didn't just draw a generic Indian pouch combining features from several different styles of pouch; OTOH, I do believe that there are some types of flapless bags that have the rear panel extend above the front and have strips of quillwork on leather applied to the rear panel, so maybe it isn't a hybrid after all. I dunno.
Interestingly enough, there is some debate among scholars of Native American art about whether those finely made beaded, fingerwoven bags and the similar leather bags with fine quillwork are typical of the more elaborate types of bags made for actual use by the Indians across the NE/Great Lakes area by or whether they were prestige items made by a handful of skilled artisans in a few centers of production and given as gifts to important chiefs, both Indian and European. Colonel Lewis here is wearing a matchcoat and quite a bit of ornamentation about his ears and hair, which makes me think that he was in "dress uniform" when the sketch was made. I'm not sure that I'd conclude that he didn't use this bag for actual use, though, as I'm not sold on the idea that Natives really made a distinction between ceremonial and fighting/using implements the way we do today. Lots of questions, not many answers.
